As tempering is the process of reheating a hardened steel up to a temperature, maximum A1, it is a low temperature process. It can be performed in a: (a) Oil-bath furnace (b) Salt bath furnace (c) Lead bath (d) Forced-air circulation furnace.
1. Oil-Bath Furnace:
Oils can be used as a medium for quick transfer of heat for tempering, or age hardening up to a temperature 250°C. Even silicone oil starts giving fumes above 200°C. These baths require special hoods to take care of fumes. A steel tank having suitable grooved refractory bricks (around it on outside) having coiled electrical resistance element placed in them, and finally enclosed by a mild steel structure, can be used for tempering in oil.
A stirring system and a suitable temperature controller can provide a uniform and constant temperature. Oils provide a bath which gives quick heat transfer, i.e., provide a high heat transfer efficiency, due to convection heating and intimate contact of oil molecules with components. The main disadvantages are- General uncleanliness; Max Temperature is 300°C. Most of them (oils) fume above 200°C. Accidental overheating causes fire.
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2. Salt Bath Tempering Furnace:
Normally for tempering, an externally heated electrical resistance salt bath is used, Suitable salt compositions are chosen to use in the range of 150°C to 600°C. Salt baths provide high heat transfer efficiency.
Mixtures of nitrates and nitrites of alkaline metals are used for baths of low melting points. Here also precautions should be taken to prevent accidental overheating, otherwise it may result in an explosion.
3. Lead Baths:
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A eutectic alloy of lead and tin, which has melting point of around 180°C, can be used for tempering above this temperature.
4. Forced-Air-Circulation Furnace:
The forced-air-circulation furnace used for tempering is usually an electrically heated type. It is well known that heat transfer at temperatures below about 700°C, is mainly by conduction and convection. The radiation plays a relatively unimportant part specially towards the lower end of this range say 150 to 400°C, in which hardened steel components are often tempered.
Since the efficiency of the tempering process depends upon a precise control of temperature and its uniformity, it is absolutely essential that the furnace should offer such a control. The normal box type furnaces have been seen to have wide difference of temperature at the hearth and as indicated by the thermocouple particularly at low temperatures.
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In order to homogenise the temperature in the furnace chamber an electrically driven fan generally of centrifugal type is used. The heating elements are so located that the charge being heated is shielded from direct radiation.
The use of a centrifugal fan with a ventuary cowling and a work chamber designed to stabilise the unidirectional flow is essential for successful operation specially where closely-packed charges of steel have to be tempered. The thermocouples are located to indicate maximum and the temperature of the charge.
When the temperatures become equal, soaking period starts. One thermocouple is at the point where hot air stream leaves the heating elements. The second thermocouple is put below the charge basket. Fig. 10.10 indicates one such furnace. It essentially consists of heat-insulated chamber, cylindrical in shape with an inner, concentric metal liner of baffle (to prevent direct radiation to the charge) to direct the air to well defined paths. Heating elements are put between baffles and wall of chamber.
The direction of air is illustrated in Fig. 10.10. The air comes in contact with hot elements to get heated and then down-flows through the charge to which heat is discharged. The charge is put in a basket with a perforated base.
